1. Field
This invention generally relates to tethered rigid buoyant platforms floating in the low stratosphere.
2. Prior Art
An area of prior art is buoyant airships and balloons that float in the atmosphere.
Balloons float freely without propulsion and are constructed from gas tight flexible membranes, usually thin plastic like polyethylene. They contain a lighter than air gas, sometimes pressurized and sometimes unpressurized. A commonly used terminology is “zero-pressure” for unpressurized balloons and “super-pressure” for pressurized balloons. Free floating balloons of both types are typically exploited for tracking weather or for scientific purposes. The largest balloons have volumes of about 1 million cubic meters and can float in the stratosphere at altitudes exceeding 40 km. They are fragile, carry small payloads and are used for one flight.
Airships have an aerodynamic shape and a means of propulsion and are categorized as rigid, semi rigid or blimps.
Blimps, like the Goodyear blimp, use a gas tight membrane filled with a pressurized lighter than air gas to provide a combination of buoyancy, structural rigidity and an aerodynamic shape. This means of construction combined with the limited strength of available membrane materials has limited the scale of blimps to a volume of a few thousand cubic meters. Blimps either have a means of propulsion or they are tethered to the ground. A tethered blimp lacking means of propulsion is usually called an aerostat. Because of their limited volume, all blimps and aerostats have been confined to the denser air environment of altitudes below 10 km in the troposphere. This is because as the air becomes less dense with altitude, a given volume provides less buoyancy. Blimps have heavy propulsion systems, fuel and passenger or equipment payloads all of which have to be supported by buoyancy. Aerostats lack propulsion systems, but have tethers that are at least as heavy. In both cases it has proven impractical to provide sufficient buoyancy or a sufficiently light blimp to enable operation in the low stratosphere.
Rigid airships are constructed with a rigid framework that provides structural rigidity and aerodynamic shape and contain zero pressure gas bags within the rigid framework to provide buoyancy. This means of construction has enabled the construction of large craft with volumes exceeding 100,000 cubic meters. Rigid and semi rigid airships have all been powered aircraft. Airships and have only operated at altitudes well below 10 km. To build airships that could operate at higher altitude involves building very much larger craft. The engineering and operational constraints of doing this combined with the lack of an economic or military demand have meant that this option has never been explored.
The earth's atmosphere in the low stratosphere in the region of 20 km altitude has benign weather properties over most of the earths surface below latitude 60 degrees that make it attractive for long endurance operation. This has been exploited by reconnaissance aircraft like the U2 and Global Hawk. Weather we are familiar with is confined to the troposphere which extends up to an altitude from about 8 km to 12 km with a gradual transition to the stratosphere called the tropopause. The high winds of the jet stream occur at the tropopause. There is no moisture or clouds in the stratosphere and turbulent weather patterns like thunderstorms and hurricanes do not reach high enough to have effect at an altitude of 20 km. This is well illustrated by flights by U2 and Global Hawk over hurricanes for weather research. Winds are steady and horizontal, mostly less than 20 meters per second, with small episodic periods in winter of a few weeks every few years where they can reach 40 meters per second due to excursions of the polar vortex which circles the poles in the stratosphere in winter.
The permanently benign weather properties of the atmosphere in the region of 20 km altitude in the low stratosphere make it a distinct and separate operational environment which enables practical long endurance operation as evidenced by the U2 and global hawk aircraft. The unique low air pressure, low air density environment requires unique aircraft designed to operate there. Conventional aircraft are designed to operate at lower altitudes up to around 12 km, and their aerodynamics and propulsion systems cannot operate at altitudes around 20 km. There have been attempts at building long endurance high altitude airships to fly at 20 km altitude and above, but none have as yet succeeded due to the difficult engineering challenges of limited buoyancy posed by the thin atmosphere. In the class of buoyant aircraft, only un-tethered and un-powered free floating weather and research balloons have operated in the stratosphere.
No prior art airship or aerostat has been designed to stay aloft on a permanent basis. Endurance is measured in weeks for airships and months for aerostats. They both have limited endurance and both must avoid bad weather.
In summary all prior art mechanisms that float in the atmosphere have been relatively small scale and short endurance and almost all have operated in the troposphere. There have been no tethered buoyant rigid structures operated in the atmosphere at any altitude.
Another feature of high altitude operation in the low stratosphere is the large distance to the horizon. From an altitude of 20 km, the horizon is approximately 550 km distant. This means that observation or communication technologies that are confined to “line of sight” operation can cover a wide area from this altitude. This includes active technologies like radars, laser and radio communications, and passive optical and radio surveillance. The air is clear at 20 km which enables uninterrupted and secure laser light communication between platforms and between platforms and spacecraft. There have been proposals for long endurance high altitude aircraft or airships to “station keep” and act as communications and observation hubs, but the operating constraints have proven too difficult. They would use solar energy during daylight hours with batteries storing energy for nighttime. Providing sufficient energy to station keep in the worst case winds of around 50 m/s has proven impractical.
Another feature of the environment in the low stratosphere is sunlight is more intense. Atmospheric scattering is much reduced due to the much smaller mass of air in the optical path, especially at lower sun elevation angles. This results in higher daily solar energy incident on a surface. This can exceed a factor of three or more times ground level solar energy at the same location depending on latitude and tracking. Also solar energy is totally predictable as it is not interrupted by weather or dust.
Photovoltaic solar energy systems use solar cells to convert solar energy directly into electricity. The solar cells are usually connected together in panels, which in turn are mounted on mechanical supports and connected together to form arrays. Associated with the photovoltaic panels are electrical elements such as conductors, voltage converters, combiners, fuses, relays surge protectors and inverters used to combine the power from the collection of PHOTOVOLTAIC panels into a single power output.
Current photovoltaic electricity systems suffer from several problems. Their high capital costs make the cost of the energy they produce uncompetitive without subsidy.
The power produced by photovoltaic panels varies by more than a factor of two depending on their geographic location. Large-scale systems in the best sunny geographic locations also have high ancillary costs to compensate for the long transmission distance from the system to the average power user.
Photovoltaic arrays need to have large entry apertures to produce meaningful amounts of power. Utility scale systems have apertures measured in millions of square meters. Current systems consequently consume large areas of land and significant quantities of construction materials like glass and steel needed to fabricate this large aperture array.
Weather in the form of dust, wind, rain, hail, frost and snow make power generation unpredictable and require that structures be strong and durable which adds significantly to their cost. Typical design wind loads are around 2000 Pa and mechanical snow loads are around 5000 Pa.
Some current large scale systems use large arrays of individually steered collecting elements. Robust mechanical support, motors, gears, electrical equipment etc are needed for each collector element, contributing significantly to overall cost.
The cost problem is compounded by the generally low overall energy conversion efficiency of current systems, which consequentially requires a larger surface area and more material to produce a given power output compared to higher conversion efficiency systems.
There have been some proposals to attach cells to tethered aerostats to generate power. These have all proposed current small scale aerostats tethered at relatively low altitudes in the troposphere. None of these proposals have been reduced to practice because of practical constraints that make them unrealistic. At all altitudes in the troposphere, weather can be severe and the durability of current aerostat technology is poor. The small scale of aerostats mean that they can at best only provide a small amount of power, and many thousands would be needed to provide power at a utility scale of hundreds of mega Watts. They would need to be spaced far apart to avoid colliding. There would be a constant need to winch them down for maintenance and to avoid weather.